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Sugar and brandy don't just make fruitcake tasty, they bestow longevity on the holiday staple. Jana Howden takes us through the chemistry of Christmas cakes.

As Christmas races towards us, plenty of people focus on food – more specifically, rich, sugary, indulgent treats, as all promises of a diet are thrown out the window.

Whether you love or hate it, fruitcake is a Christmas staple – and some people really have a thing for it. In 2011, a 70-year-old fruitcake sold for more than US$500 – but the amazing part is it may have still been edible.

Fruitcakes are said to improve with age, like a good wine. Fruit skins contain biomolecules known as tannins, the same compounds that enhance flavours of an aged red. The longer tannins sit in fruitcake – or wine – the deeper the complexity of flavour.

But old fruitcake is still edible largely thanks to the cake’s high sugar content. While it sounds paradoxical, it actually prevents microbial growth.

Say a bacterial cell such as Salmonella, which is typically 80 to 90% water, finds itself in a fruitcake.

The concentration of sugar in the surrounding cake is much higher than the solutes in the cell, so in an effort to balance this difference, water will drain of the cell and into the sugary cake – killing the bug. (This osmotic pressure also explains how salt preserves food.)

Additional protection comes from the dense nature of fruitcake, which largely prevents air filtering through. This restricts the growth of bacteria that need oxygen to survive.

And if the cake is doused in brandy or any other strong alcohol, the high ethanol content of the alcohol adds another layer of defence against microbial spoil.

Despite fruitcake’s apparent invincibility, the ageing process can cause the cake to dry out and become stale. Cake flour contains starch molecules which naturally form rigid crystalline structures.

When a cake is baked, these formations melt and become pliable; this is called starch gelatinisation. But over time, the starch molecules shuffle back into their crystallised arrangement, hardening the once-fresh cake.

Luckily (or unluckily, for those of you hoping this would grant you an excuse to throw out last year’s fruitcake) this staling process can be easily reversed.

According to physicist Peter Barham, who spoke to the American Physical Society on the subject, simply wrap an old fruitcake in aluminium foil and bake it in the oven for a few minutes. This re-gelatinises the starch and freshens the cake.

With reports of a single fruitcake being passed down one family for more 130 years, take comfort in the knowledge that you can keep putting off your yearly slice of fruitcake “until next year” for years to come.